Paint Evidence: Layers, Coating Structure and Instrumental Analysis

A paint film is a cured polymer matrix with dispersed solid and liquid components. Four ingredient classes do the work:

• Binder (resin). The polymer that cures from a liquid film into a solid coating. Alkyd, acrylic, polyurethane, epoxy, polyester, melamine cross-linked variants. The binder is the largest single contributor to the FTIR spectrum and the first thing the lab identifies.
• Pigment. Insoluble fine particles that provide colour and opacity. Titanium dioxide (rutile or anatase) for white and as an opacifier in most colours, iron oxides for red and brown, carbon black, chromium oxides for green, ultramarine and phthalocyanine blues, lead chromate (rare in modern OEM paint, still present in older Indian commercial vehicles), organic colorants. Pigments give the SEM-EDX elemental fingerprint.
• Solvent. The volatile carrier that allows application; not present in the cured film. Identifying the original solvent is rarely useful in casework because it has evaporated by the time the lab sees the sample.
• Extender (filler). Inexpensive inorganic particles (calcium carbonate, talc, kaolin, barium sulfate) that add bulk and modify rheology. Show up clearly on SEM-EDX as Ca, Si, Mg, Ba peaks.
• Additives. Small percentages of UV stabilisers, anti-foaming agents, levelling agents, driers. Rarely diagnostic on their own; can become diagnostic when an unusual additive is present.

The three families of paint that turn up in Indian SFSL casework break down by where the paint was applied and how many layers it carries.

A scene examiner who can identify the paint family under a stereomicroscope at 10 to 40x is performing preliminary triage that the lab will confirm at intake. Architectural paints are short stacks of similar layers. Automotive OEM is a four-or-five-layer sandwich with a clear, structured sequence. Refinish work is a shorter stack laid over an existing OEM stack, often with visible boundary contamination. Industrial paints are usually pigmented monocoats or two-layer systems over zinc-rich primer.

A modern Indian-market passenger car (post-2010 Maruti, Hyundai, Tata, Mahindra factory production) carries a paint system applied in a fixed sequence on the body-in-white line. Each layer has a specific function and a specific chemistry, and reading the layers in cross-section under a stereomicroscope is the first thing the SFSL Trace division does with a recovered flake.

Substrate up, the layers are: bare metal, e-coat, primer surfacer, basecoat, clearcoat.

• E-coat (electrocoat primer). Applied via cathodic electrodeposition: the body shell is dipped in a tank of charged epoxy resin and the resin migrates onto the metal under voltage. Gives uniform thickness even in cavities, near-universal corrosion protection. Almost always black, almost always epoxy. About 20 µm thick.
• Primer surfacer. Sprayed over the e-coat to level surface defects and provide stone-chip resistance. Polyester or polyurethane, often grey or a colour matched to the basecoat to improve coverage. About 30 µm thick.
• Basecoat. The colour layer. Acrylic-melamine in older systems, polyurethane in newer ones. Contains the colour pigments and any metallic or pearlescent flakes. About 15 µm thick when applied as a single coat; thicker when applied as a wet-on-wet two-coat system for some metallics.
• Clearcoat. Unpigmented topcoat for gloss, UV resistance, and chemical durability. Acrylic-urethane in most modern Indian-market cars. About 40 µm thick. The clearcoat is what the FTIR most easily distinguishes because it is the only layer that is reliably free of pigment interference.

Refinish work shortcuts this sandwich. A typical body shop repaint over an existing OEM stack adds a primer (sometimes), a basecoat, and a clearcoat, for a total layer count of 7 to 9 in cross-section. The boundary between the original OEM clearcoat and the refinish primer is usually visible under 100x as a sharp interface with trapped contamination (dust, sanding residue). Indian appellate courts have accepted refinish-vs-OEM determinations made on these stratigraphic grounds.

Paint analysis at an Indian SFSL is sequenced from least to most destructive. The first pass is microscopy, the second is FTIR, the third is Py-GC-MS or SEM-EDX depending on the question. The sample is irreplaceable, and consuming it with a thermal technique before microscopy has irreversible evidentiary consequences.

1. Stereomicroscope examination, 10 to 40x
   Count layers in cross-section, note colours, identify gross features (metallic flakes, mica platelets, refinish boundaries). Document with calibrated photomicrographs. No sample loss.
2. Cross-section preparation
   Embed the flake in epoxy resin, cure, microtome at 5 µm, mount on a microscope slide. The cross-section becomes the analytical platform for everything downstream.
3. FTIR-ATR (binder identification)
   Press the cross-section against the ATR crystal or pick individual layers with a tungsten needle for transmission FTIR. The binder spectrum is diagnostic: polyurethane, acrylic, alkyd, epoxy each have distinctive carbonyl and C-O-C signatures. Sample loss is microgram-scale.
4. SEM-EDX (pigment and extender elements)
   Image the cross-section and run point-EDX on each layer. Ti for white pigment, Fe for red/brown, Pb (rare in modern paint) for older Indian commercial vehicles, Ca/Mg/Ba for extenders. Layer-by-layer elemental fingerprint.
5. Py-GC-MS (definitive binder ID)
   When FTIR alone is ambiguous, pyrolyse a microgram of the layer at 500 to 700°C; separate the fragments by GC; identify by mass spectrum. Discriminates between subtypes of acrylic, polyurethane and polyester that FTIR cannot resolve. Sample loss is significant; reserve for when discrimination demands it.
6. PDQ database query (automotive cases only)
   Submit layer sequence, layer thickness, binder identifications and pigment elemental data to the PDQ database via the RCMP cooperation channel. Receive a candidate make/model/year/region list. Indian SFSLs use this on flagship cases only because the bilateral process takes weeks.

A worked timing for a routine hit-and-run paint flake at a well-resourced Indian SFSL: microscopy and cross-section preparation in day 1; FTIR in day 2; SEM-EDX in day 3 or 4; Py-GC-MS only if FTIR is ambiguous, in week 2. Report drafted week 2 or 3. PDQ query, when triggered, adds another 3 to 6 weeks. State SFSLs without Py-GC-MS in-house refer that step to CFSL Chandigarh or Hyderabad.

Each instrument answers a specific question and has a defined limit. Understanding that boundary is essential for accurate expert testimony.

• FTIR (Fourier Transform Infrared Spectroscopy). Identifies the binder by its absorption pattern in the mid-IR (4000 to 400 cm⁻¹). Two modes: ATR (attenuated total reflectance, pressing the sample against a diamond or germanium crystal, fast and near-non-destructive) and transmission (picking out a thin layer with a tungsten needle, more sensitive, slower). Polyurethane shows a strong N-H around 3300 cm⁻¹ and C=O around 1720 cm⁻¹. Acrylic shows ester C=O around 1730 and aliphatic C-H stretches. Alkyd shows ester C=O around 1730 plus aromatic absorption around 1600 and 1580. The technique is the workhorse; what it cannot do is distinguish subtle differences between polyurethane formulations from different manufacturers.
• Py-GC-MS (Pyrolysis-GC-MS). Heats a microgram of paint to 500 to 700°C in an inert atmosphere; the polymer decomposes into characteristic fragments; the fragments are separated by GC and identified by mass spectrum. Distinguishes between subtypes of acrylic, polyurethane and alkyd that share an FTIR class signature. It is the technique employed when FTIR returns the same binder class on two samples and the examination requires discrimination at the subtype level.
• SEM-EDX. Scanning electron microscope with energy-dispersive X-ray spectroscopy. Gives a high-magnification image of the cross-section plus point elemental analysis on each layer. Reports the major elements (Ti, Fe, Si, Ca, Al, Mg, Ba, S) and several minor ones (Pb, Cr, Zn, Cu) at percent or tenth-of-percent level. The technique that picks up extender packages (Ca + Mg + Si pattern points to a specific filler blend) and the rare-element fingerprints that escape FTIR.
• XRF (X-ray fluorescence). Used as a screening tool at some state SFSLs. Less sensitive than SEM-EDX but non-destructive and faster, useful for triage when many samples have to be sorted before deeper analysis.

Paint collection follows a small set of rules that prevent the most common evidentiary failures. The principles are summarised in the broader scene-processing workflow; the paint-specific points sit below.

• Flakes. Recover with clean forceps onto a folded paper packet, then into a paper envelope. Never plastic, which carries static charge and can pull the flake out of the packet during handling. Never adhesive tape directly to the flake, which contaminates the layer surface with adhesive that interferes with FTIR.
• Smears. Paint smears on victim clothing get the cloth itself packaged in paper; the lab will scrape the smear under stereomicroscope at intake. Do not attempt to scrape at the scene; loss of layer information is irreversible.
• Control samples. For a suspect vehicle, take paint chips from every panel of the same colour: bonnet, roof, doors, bumpers (if painted), tailgate. Each panel is a separate control with its own envelope. Different panels can carry different repair histories and therefore different layer sequences, and the defence will exploit a single-panel control on cross-examination.
• Roadway samples. A paint flake on a road surface at a hit-and-run scene gets photographed in situ with scale, then lifted with clean forceps. Note the spread pattern; like glass, paint spread direction indicates direction of vehicle travel.
• Hair, clothing, skin. Tape-lift onto paper backing. The lab will sort under stereomicroscope.

The Indian hit-and-run workflow ties paint and glass together. A typical case: a pedestrian fatality on a state highway, no eyewitness, a windscreen-glass cluster and a paint smear at the scene. SFSL findings: glass RI 1.518, density 2.501 g/cm³, ICP-MS trace profile narrows to a Toyota Innova production batch; paint flake recovered from the victim's jacket shows four-layer OEM stack with polyurethane clearcoat, acrylic-melamine basecoat in magnetic gray metallic, polyester primer surfacer, black epoxy e-coat; PDQ query returns Toyota Innova 2018 to 2021 in Toyota Bidadi paint specification 1G3. A vehicle matching the description is recovered three days later from a service station; physical fit on the glass plus stratigraphic correspondence on the paint plus matching basecoat pigment chemistry carries the conviction.

The PDQ (Paint Data Query) database deserves a separate note. Maintained by the RCMP in Ottawa, PDQ catalogues automotive paint systems from OEMs worldwide by layer composition, layer count, binder chemistry and pigment elemental data. Indian SFSLs query PDQ through bilateral arrangements on flagship cases (terror-related vehicular cases, cross-border smuggling, high-profile hit-and-run prosecutions). A domestic Indian equivalent has been discussed at NFSU and the Bureau of Police Research and Development for several years; as of mid-2026 the public position is that a national paint database is in scoping phase, not yet operational. Until it is, Indian SFSLs match against in-house reference collections (typically a few hundred samples per major lab, biased to vehicles common in regional casework) and against PDQ on a case-by-case basis.